1. Field of the Invention
The present invention relates to a constitution of an active matrix liquid crystal display device. It also relates to a process for fabricating the same.
2. Description of the Related Art
Heretofore, there has been known an active matrix liquid crystal display device. This has a structure in which thin film transistors are disposed on respective pixels being arranged in a matrix, so that electric charges which enter into and outgo from the pixel electrodes can be controlled by thin film transistor.
The constitution above requires a use of a light shielding film that is provided in such a manner to cover the edge portions of the pixel electrodes, and is called as a “black matrix (BM)”. In general, a metallic film provided at a thickness of several thousand of angstroms (Å) is used as a BM.
The black matrix does not particularly function electrically, but it is present over the entire pixel matrix region. However, the presence of a thin metallic film being interposed between insulating films and on the entire pixel matrix region induces a problem of accumulating unnecessary charges therein. This problem is not specific after the completion of the device, but is also found in the fabrication process thereof.
As is well known, a film forming step or an etching step using plasma is employed in a general process of fabricating a thin film transistor. If a conductive material exists electrically floating in the fabrication process above, electric charges would be accumulated therein to cause an electrostatic breakdown to an insulating film.
A generally used insulating film is several thousand of angstroms (Å) in film thickness. Further, defects and pinholes are present inside an insulating film (a silicon oxide film or a silicon nitride film) at a non-negligible density.
Accordingly, electrostatic breakdown occurs locally on the insulating film as a result of the phenomenon of charge accumulation in BM above.
This signifies that a failure occurs partially on the device during the fabrication process. That is, the thin film transistors may partially malfunction or the circuits may cause operation failure due to the presence of leak current.
The problem above is particularly serious in the course of fabrication process. Also, after completion of a device, such a problem is a factor of losing reliability of the device.
In the light of the above-mentioned circumstances, an object of the present invention is to overcome the above-mentioned problem of charge up of the black matrix. More specifically, an object of the present invention is to suppress the generation of failure which occurs during the fabrication process due to the charge up of the black matrix, and to thereby improve the reliability of a device after completion.